Antenna for a backscatter-based RFID transponder
View Patent ↗An antenna is provided for a backscatter-based RFID transponder with an integrated receiving circuit, having a capacitive input impedance, for receiving a radio signal lying spectrally within an operating frequency range, whereby the antenna has two antenna arms, which extend outwardly in a spiral from a central area, in which the antenna arms can be connected to the integrated receiving circuit. According to the invention, each antenna arm has an arm length along the arm, which is selected so that one of the series resonance frequencies of the antenna is below the operating frequency range and the next higher parallel resonance frequency of the antenna is above the operating frequency range. The invention relates furthermore to a backscatter-based RFID transponder with an antenna of this type.
1. An antenna for a backscatter-based RFID transponder comprising:
an integrated receiving circuit having a capacitive input impedance for receiving a radio signal lying spectrally within an operating frequency range; and
two antenna arms that extend outwardly in a spiral from a central area in which the antenna arms are connected to the integrated receiving circuit,
wherein said antenna arms together exhibit a plurality of resonance frequencies including a first plurality of series resonance frequencies when a reactance of said antenna arms passes from a capacitive reactance to an inductive reactance and a second plurality of parallel resonance frequencies when said reactance passes from an inductive reactance to a capacitive reactance,
wherein each antenna arm has an arm length along the arm, which is selected so that one of the series resonance frequencies of the antenna is below the operating frequency range and the next higher parallel resonance frequency of the antenna is above the operating frequency range.
2. The antenna according to claim 1 , wherein the arm length is selected so that the antenna has values of an inductive input impedance, which within the operating frequency range are approximated to conjugate complex values of the capacitive input impedance in such a way that no circuit arrangement for impedance matching is necessary between the antenna and integrated receiving circuit.
3. The antenna according to claim 1 , wherein the arm length is selected so that the series resonance frequency that results in the antenna having values of an inductive impedance, which in the operating frequency range are approximated to the conjugate complex values of the capacitive input impedance in such a way that no circuit arrangement for impedance matching is necessary between the antenna and integrated receiving circuit, is below the operating frequency range.
4. The antenna according to claim 1 , wherein the series resonance frequency corresponds to the lowest series resonance frequency of the antenna.
5. The antenna according to claim 1 , wherein each antenna arm is formed to circumscribe at least one full turn or at least 1.5 full turns, around the central area.
6. The antenna according to claim 1 , wherein each antenna arm has an arm width transverse to the arm, and wherein the arm width changes along the arm.
7. The antenna according to claim 6 , wherein the arm width increases outwardly proceeding from the central area.
8. The antenna according to claim 1 , wherein each antenna arm forms an inner radial spiral and an outer radial spiral.
9. The antenna according to claim 8 , wherein the inner radial spiral and the outer radial spiral follow a logarithmic function.
10. The antenna according to claim 1 , wherein the antenna arms are made polygonal or straight piece-wise.
11. The antenna according to claim 10 , wherein the arm width along the arm is constant piece-wise.
12. The antenna according to claim 1 , wherein the antenna arms are made identically in their outer form.
13. The antenna according to claim 1 , wherein the antenna arms are made planar and lie in a common plane.
14. The antenna according to claim 1 , wherein each antenna arm comprises a thin conductive layer, which is formed on a substrate.
15. The antenna according to claim 1 , wherein the two antenna arms are formed on the integrated receiving circuit.
16. A backscatter-based RFID transponder comprising:
an integrated receiving circuit having a capacitive input impedance; and
an antenna connected to the integrated receiving circuit, the antenna comprising:
two antenna arms that extend outwardly in a spiral from a central area in which the antenna arms are connected to the integrated receiving circuit, wherein said antenna arms together exhibit a plurality of resonance frequencies including a first Plurality of series resonance frequencies when a reactance of said antenna arms passes from a capacitive reactance to an inductive reactance and a second plurality of parallel resonance frequencies when said reactance passes from an inductive reactance to a capacitive reactance, and each antenna arm having an arm length along the arm, which is selected so that one of the series resonance frequencies of the antenna is below the operating frequency range and the next higher parallel resonance frequency of the antenna is above the operating frequency range.
17. The backscatter-based RFID transponder according to claim 16 , wherein the integrated receiving circuit is disposed in the central area of the antenna arms.
18. The backscatter-based RFID transponder according to claim 16 , wherein each antenna arm comprises a thin conductive layer, which is formed on a substrate, and wherein the integrated receiving circuit is formed on the substrate.
19. The backscatter-based RFID transponder according to claim 16 , wherein the capacitive input impedance has an effective resistance and a reactance, a value of the reactance being higher amount-wise than a value of the effective resistance.
20. The backscatter-based RFID transponder according to claim 16 , wherein the transponder is passive or semi-passive.
21. The backscatter-based RFID transponder according to claim 16 , wherein the operating frequency range is within the UHF or microwave frequency range.